1. Field of the Invention
The present invention relates to a signal processing apparatus, an information processing apparatus, a signal processing method, a data display method, and a program.
2. Description of the Related Art
A variety of electronic devices (such as car navigation equipment, mobile phones, and digital still cameras) are now being equipped with positioning functions that utilize the Global Positioning System (GPS). Typically, when utilizing GPS in an electronic device, a GPS module receives signals from four or more GPS satellites, the device's position is measured on the basis of the incoming signals, and the user is informed of the measurement results via the screen of a display apparatus or similar component. More specifically, the GPS module demodulates the incoming signals to acquired orbital data for each GPS satellite, and then uses a system of equations to derive the three-dimensional position of the device from the orbital data, time information, and delay times of the incoming signals. Signals are received from four or more GPS satellites in order to eliminate the effects of clock error between the module and the satellites.
Herein, a signal transmitted from a GPS satellite (L1-band, C/A code) is a spread spectrum signal wherein 50 bps data has been spread using Gold code with a code length of 1023 and a chip rate of 1.023 MHz, and furthermore wherein the signal has been Binary Phase Shift Keying (BPSK) modulated using a 1575.42 MHz carrier. Consequently, reception of the above signals from GPS satellites by the GPS module involves spreading code, carrier, and data synchronization.
Generally, a GPS module provided in an electronic device first frequency-converts the carrier frequency of an incoming signal to an intermediate frequency (IF) of several MHz, and then conducts synchronization and other processing. A typical intermediate frequency may be 4.092 MHz, 1.023 MHz, or 0 Hz, for example. Normally, the signal strength of an incoming signal is smaller than the signal strength of thermal noise, with the S/N ratio falling below 0 dB. However, demodulating the signal is made possible by the process gain of spread spectrum techniques. In the case of a GPS signal, the process gain with respect to a 1 bit data length may be (10*log [1.023 MHz/50]), or approximately 43 dB.
As described above, the market for electronic devices equipped with a GPS module is growing. On the performance side, signal sensitivity is being enhanced, and GPS modules having signal sensitivities between −150 dBm to −160 dBm are becoming common. However, as GPS modules are becoming more widespread, the electronic devices equipped with GPS modules are also increasing in performance. The unwanted electromagnetic radiation that emanates from the electronic device as a result becomes noise, and in a growing number of case, the inherent performance of the module is not experienced. Noise emanating from the electronic device can be caused by various factors, such as internal couplings in the wiring of the electronic device, a clock that interferes spatially, the harmonic components of high-speed signals passing through a data bus or similar component, circuit load fluctuations, and power fluctuations by a switching regulator.
If the external noise described above is introduced into the analog circuits of the GPS module from the electronic device, then signal sensitivity is degraded. Such degradation in signal sensitivity does not pose a problem is the signal strength of the external noise is less than or on the order of the signal strength of the steady thermal noise produced by the GPS module (approximately −111 dBm when computed at 2 MHz bandwidth). However, when the signal strength of the external noise approaches and exceeds the signal strength of the thermal noise, signal sensitivity degrades to the extent that the level of the steady thermal noise is exceeded. Furthermore, if the inverse ratio of the incoming signal versus the sum of the thermal and external noise (hereinafter, S/(N+I)) approaches the process gain, GPS signals might no longer be detected. Even in the case where the inverse of S/(N+I) is sufficiently smaller than the process gain, the thermal noise and the GPS signal will be constrained if the voltage value in the circuit is saturated by strong external noise, for example. As a result, signal sensitivity drops sharply. Particularly, the total amplification is 100 dB or more in the case of a typical GPS module, while the resolution of analog-to-digital (AD) conversion is 1 or 2 bits. In this case, positioning is basically carried out in a state where the thermal noise and the GPS signal are saturated to some degree. For this reason, if external noise with a high signal strength is input, then the AD-converted output signal ultimately output by the analog circuit will be readily saturated.
Consequently, in order to efficiently elicit the performance of a GPS module provided in an electronic device, there is a demand for countermeasures against noise, such as the unwanted radiation emanating from the electronic device. For example, a shielding material or shielding case might be used. As another example, features such as the circuit board structure, antenna shape, and layout of elements may be optimized during the design of the electronic device, such that noise pickup by the antenna is minimized. These countermeasures can therefore affect the design, cost, and development period of electronic devices.
Consequently, a noise rating apparatus has been proposed, able to quantitatively rate noise with high precision by weighting the levels of noise entering a GPS module according to frequency (see, for example, Japanese Patent No. 4060038). Additionally, there have been proposed methods for detecting anomalous level assumed to noise by using, for example, the correlation between the C/A code of a non-existent satellite and an IF signal (see, for example, Japanese Patent No. 3949576, and Japanese Unexamined Patent Application Publication Nos. 2007-78703 and 2000-249754).